Embedded Files
objectives
objectives
Specific Heat Capacity
Specific Heat Capacity
- If a material absorbs thermal energy, then unless it is melting or boiling, its temperature rises.
- However, some materials have a greater capacity for absorbing thermal energy than others.
- For example, if you heat a kilogram each of water and aluminium, the water must be supplied with nearly five times as much energy as the aluminium for the same rise in temperature:
- 4200 joules of energy are needed to raise the temperature of 1 kg of water by 10C.
- 900 joules of energy are needed to raise the temperature of 1 kg of aluminium by 10C.
- Scientifically speaking, water has a specific heat capacity of 4200 J/(kg °C). Aluminium has a specific heat capacity of only 900 J/(kg 0C). Other specific heat capacities are shown in the table below left.
- The energy that must be transferred to an object to increase its temperature can be calculated using this equation:
- In symbols:
- where m is the mass in kg, c is the specific heat capacity in J/(kg 0C), and ΔT represents the temperature change in °C (or in K).
- The same equation can also be used to calculate the energy transferred when a hot object cools down.
- In this case, the temperature change is 50 °C.
So: energy transferred = mcΔT = 2 X 4200 X 50 J
= 420 000 J
A table showing the Specific Heat Capacities of different materials
Thermal Capacity
Thermal Capacity
- The quantity mass X specific heat capacity is called the thermal capacity (or heat capacity). For example, if there is 2 kg of water in a kettle: thermal capacity of the water = 2 kg X 4200 J/(kg °C) = 8400 J/ °C
- This means that, for each 1°C rise in temperature, 8400 joules of energy must be supplied to the water in the kettle. A greater mass of water would have a higher thermal capacity.
Measuring Specific Heat Capacity
Measuring Specific Heat Capacity
Water
- A typical experiment is shown on the right. Here, the beaker contains 0.5 kg of water.
- When the 100 watt electric heater is switched on for 230 seconds, the temperature of the water rises by 10 °C.
- From these figures, a value for the specific heat capacity of water can be calculated:
(Omitting some of the units for simplicity)
- energy transferred to water = mcΔT - 0.5 X c X 10
- energy supplied by heater = power X time = 100 X 230 = 23 000 J
- so: 0.5 X c X 10 = 23 000
- Rearranged and simplified, this gives c = 4600
- so the specific heat capacity of water is 4600 J/(kg °C).
- This method makes no allowance for any thermal energy lost to the beaker or the surroundings, so the value of c is only approximate.
Aluminium
- Aluminium (or other metal) The method is as previous, except that a block of aluminium is used instead of water.
- The block has holes drilled in it for the ; heater and thermometer. As before, c is calculated from this equation:
- power X time = mcΔT (assuming no thermal energy losses)
A video revising specific heat capacity
A video revising specific heat capacity
5.10---Specific-Heat-Capacity.mp4Storing Thermal Energy
Storing Thermal Energy
- Because of its high specific heat capacity, water is a very useful substance for storing and carrying thermal energy.
- For example, in central heating systems, water carries thermal energy from the boiler to the radiators around the house.
- In car cooling systems, water carries unwanted thermal energy from the engine to the radiator.
- Night storage heaters use concrete blocks to store thermal energy. Although concrete has a lower specific heat capacity than water, it is more dense, so the same mass takes up less space.
- Electric heating elements heat up the blocks overnight, using cheap, ‘off-peak’ electricity supplied through a special meter.
- The hot blocks release thermal energy through the day as they cool down.
- In most central heating systems, water is used to carry the thermal energy.
Specific Heat Capacity Worksheet
Specific Heat Capacity Worksheet
5.10---Specific-Heat-Capacity.docxPage updated
Report abuse